Electronic microscope



Dec. 15, 1942. Eg-RUSKA El' AL ELECTRONIC MICROSCOPE I Filed Feb. 28, 1940 2 Sheets-Shet 1 Dec. 15, 1942. E. RUSKA ETAL ELECTRONIC MICROSCOPE Filed Feb. 28, 1940 2 sheets-snee 2 Patented Dec. 15, 1942 ELECTRONIC MICROSCPE Ernst Ruska, Beriin-Spandau', Emil Hentschel,

Falkensee, Kreis Osthavelland,

and Hans Schuchmann, Berlin-Spandau; Germany; Vest ed in the Alien Property Custodian Application February 28, 1940, Serial No. &21,240

In Germany February 21, 1939 v 11 Ciaims. (CI. 250-495) The present invention relates to electronic microscopes, and more particularly to an electron emitting source for high direct voltages such as required for electronic microscopes. k

It is an object of the invention to provide an eicient electron-emitting source for electronic microscopes which ensues and facilitates: an exact centering of the Component parts so that the center of the emitting surface of the incandes- 'cent cathode, forinstance the tip of the hairpin shaped cathode, the center of the aperture in the control diaphragm and the center of the anode aperture coincide very accurately, at least within %a mm., with the optical axis of the apa paratus. Another .object of the invention is to ensure very steady operation of the glow cathode of the electron source. An object also is to prevent the occurrence of spray discharges along the surfaces of the exterior parts of the electron emitting source as well as undesired discharges in the interiorof the apparatus. The invention further aims at meeting these requirements while providing completely unobjectionable inner and outer insulation for high Operating voltages of,

end, a cone concentrical to the bore for mounting the insulator on a corresponding conical part of the anode. The insulator and the cathode structure also engage each other with conical contact surfaces. Such a construction affords a very accurate centering of the main parts of the electron emitting apparatus.

The anode serving as a support for the abovementio-ned insulator comprises, according to another feature of the invention, two coaxial anode cylinders, the outer cylinder having an inner con ical surface tting the corresponding conic'alsur face of the insulator. The arrangement is preferably so chosen that the end of the insulator facing the anode projects into the space free of magnetic fields iormed between the two anode cylinders. In this manner the acceleration space may be so designed as to prevent the occur-rcnce of tangential field strengths along the innerwalls of the insulator tube, which at higher Operating voltages may lead to discharges. The lines of force emitting from the cathode towards the in- I In the portion of the vacuum chamber between the cathode and the anode, in 55 'which magnetic fields prevail, the distances are to be kept as small as possible and the electrodes must be properly polished to prevent are discharges. The openings necessary for the evacuation are preferably arranged at the bottom of `the circular space between thetwo anode cylinders, i. e., in the space free of 'magnetic fields, in order to avoid long ion starting paths which would cause flash overs.

It is preferable to arrange and design the sealing cones between the cathode shaft and the insulator and between the insulator. and the anode in such a manner that-the scaling grease is substantially prevented from penetrating into the vacuum chamber. To this end, the scaling cones 'maybe provided with grooves in which the scaling grease is collected during the normal operating period of a cathode until the cathode is replaced by another one. v

Owin to the fact that the vacuum maintainable in an electron microscope partly sealed by rubber rings is only moderate, for instance about 10- mm., the life of a hot cathode is limited because of the impinging thereon of ions. It is therefore necessary to replace the cathode of "electronic miscoscopes after a given period of operation. Hen'ce, an object of the invention also is to design the electron-emitting apparatus in such a manner as to enable a replacement-of the cathode with the simplest means 'and to ensure again an exact centering of the substituted cathode. To this end, according to the invention, the cathode proper consisting preferably of a tungsten wire in the form of a hairpin is mounted on a cylindric insert body detachably arranged in the cylindrical bore of a cap-shaped diaphragm. Since wire cathodes, when being heated up for the first time and also afterwards, change their form and are thereby decentered, they are heated up according to the invention under vacuum for a few minutesbefore they are centered and inserted in theelectronic microscope, the preheating being effected at a current intensity higher than that in normal operation. After'such heat r treatment, the tip of the cathode wire is correctly centered with respect to the cylinder surface of its insert body this centering is done outside of the electron microscope. The cathode proper is.

secured to an insulating base, preferably of glass or ceramic material, and this base is so designed that it may be shifted relatively to the cylindric surface of the insert body with the aid of three set screws spaced from one another 120', one of which may be substituted by a counter-spring.

The invention also provides means for efiecting proper relatively to the body. The cathode in-` serts can be supplied from the manufacturer to the customer in a protective box, for instance, in a preheated and centered state so that the inserts are ready to be inserted in the electron microscope.

The foregoing and other objects and `features of the invention will be readily understood from the following description of the accompanying drawings showing an embodiment of the inven tion in diagrammatic form. I i

Fig. l shows a 'vertical sectonal view of the upper part of the electronic microscope;

Fig. 2 shows on ,a larger scale a vertical sectional view of the same cathode shaft as illustrated in Fig. l yet removed from the rest of the microscope structure;

Fig. 3 shows a sectional view, and

Fig. 4 a plan View of the cathode insert used in the microscope of Figs. 1 and 2; and

Fig. 5 is a View of an auxiliary centering device partly in section, while Fig. 6 is a plan view thereof.

Referring to Fig. 1, the radiating apparatus of the electron microscope comprises a cathode structure, an anode structure, and a hollow insulating body, designated by ll, which encloses the cathode structure and connects it mechanically with the anode structure. More in detail, 'I is a cylindric shaft-like enclosure of the cathode structure l denotes the point of the tungsten wire cathode, and 2 a cap-shaped control diaphragm having an aperture 3. The anode structure comprises two co-axial anode tubes, i. e. a short inner tube 4 and a longer outer tube 5. 6 denotes the anode aperture. The cathode shaft 1 which is provided at its upper end 'with a sleeve 8. This sleeve carries a part 9 provided with an inner cone which contacts with a corresponding outer cone li) of a spreader insulator li. I'he lower end of the spreader insulator is provided with an outer cone and fits in the corresponding inner cone of the anode part 5. To attain an 'accurate centering of the parts ll and 6 all conical contact surfaces of the parts which support the cathode in spaced relation to the anode are accurately machined to gauge. The conical contact surface of the anode part 5 is interrupted by grcoves 29 serving to collect the grease which during the operation tends to enter the vacuum chamber. The lower end of the insulating body I! extends into the space between the cathode parts 4 and 5 which is free of magnetic flelds. 30 denotes a passage which opens into the space free of magnetic fields and communicates with the lower space of the electronic microscope connected to a vacuum pump (not shown) so that the inner space of the electron emitting source is also under the efiect'of the vacuum pump. !3 designates the ribs of the insulator which ensure a sufiicient' surface leakage path for insulating the high voltages employed in the -arrangement Y The 'anode part 5 is integral 'with the inner iron jacket !4 of the condenser coil l5. !5 is the upper part of the microscope portion which accommodates the object carrier (not shown). This part 16 has a conical, ground surface ll engaging the corresponding conical portion of a support le. The support !8 is provided with a ball bearing 20 having three balls spaced at an equal distance from one another and supporting a holder [9 movable relatively to the support e. The holder !9 has a spherical surface as indicated at 2! whose center lies in the plane of the object. Against this surface z bears the lower part l5l of the condenser lens through a ball bearing 22, also with three balls spaced at an equal distance from each other. 23 denotes pressure screws secured to the upward extending part 24 of the holder 13 and hearing against the enclosure` l52 of the condenser coil [5. Opposite to the pressure screws 23 are arranged resilient holders 25. By adjusting the screws 23, the condenser coil may be shifted on the spherical surface z together with the entire electron emitting apparatus. In this manner the angle is varied at which the electron beam strikes the object. 25 are screws Secured as shown in the drawings to the support la and hearing against the holder [9. Opposite to the screws 26 are arranged corresponding reslient holders 21. By adjusting the screws 26, the holder !9 may be shifted in a plane perpendicular to the direction of the beam together with the condenser coil and the electrcn emitting apparatus. By this adjustment it' is possible to Shift the electron beam in parallel relation to the axis of the microscope. While the drawings show only one screw 23 and one counteracting spring 25, it is to be understoodthat a second screw and spring are provided and spaced from the screw and spring illustrated so that the radiating apparatus may be adjusted to any angular position with the limits of the construction. Likewise a second 'screw 25 and counteracting spring 21 are provided between support !8 and holder [9 and spaced 90 from the screw and spring illustrated, in order to permit sh'fting the radiating apparatus in any direction transversal to the beam. The parts 15 and l8 of the arrangement, movable relatively to each other, are sealed by a rubber sleeve 28.

As will be seen from Fig. 2, the sleeve 8 and the diaphragm cap 2 engage the shaft 'I at cylindrical and concentrical contact surfacesgl and 32 respectively. This design ensures that the main parts composing the electron-emitting apparatus are practically coaxial with the optical axis of the electron microscope so that only very slight deviations are still to be corrected. 33 is a nut with the aid of which the upper end of the shaft 'lis Secured to the sleeve B. The current supply conductors 34 and 35 leading to the cathode extend through the cylindrcal inner space of the shaft 1. These conductors are held in position with the aid of insulating holders 36 and 31. Sleeves 38 and 39 are screwed on the threaded upper ends of the conductors 34 and 35 respectively. The free space 40 is filled with a suitable insulating substance after securing the shaft 1 to the sleeve a. i i

Two sleeves 41 and 42 respectively are screwed on the lower threaded' ends of the conductors 34 and 35. 43 is a cap screw,'with the aid of which thediaphragm cap 2 is secured to thegcathode shaft 1. The tungsten cathode 44 is sealed in a glass member 45 Secured to an insulating body 45. Instead of the glass member, a ceramic base may also be employed for holding the tungsten cathode. The insulating body 46 contains two tubes 41 and 48 through which extends the tungsten wire. The ends or the tungsten wire are soldered, as indicated at 49 and 50, to the ends of the tubes. The plug pins 41 and 48 thus formed are inserted in the sleeves 4! and 42 respectively in the manner shown. The cathode insert has an outer metallic jacket l, preferably of bronze, which fits the cylindrical diaphragm cap 2. 'The inner space of the shaft 'l for the reception of the current supply conductors 34, is in .open communication through openings 52 with the vacuum chamber of the arrangement.

As mentioned above, the glass member may be shifted relatively to the cylindrical outer jacket 5! of the insert in order to enable an exact centering of the cathode point I with respect to this cylindrical contact surface. Figs. 3 and 4 show three set screws 53 spaced 120 from one another, by means of which the centering may be efiected. Instead of the three set screws, also two set screws spaced 120 from each other and a counter-pressure spring also spaced 120 from the set screws may be employed. The exact centering is preferably effected with the aid of a light optical microsccpe. In Figs. 5 and 6, SI denotes the object table of such a light optical microscope, 62 is a holder concentric with the optical axis of the microscope. The bore !62 of the holder fits the jacket of the cathode insert 5! (Figs. 2 and 3). The centering device is provided with three screw drivers 63 displaced 120 from each other and arranged at a height corresponding exactly to the slots of screws 53 (Figs. 3 and 4) arranged in the insert. The point l of the cathodeis shifted with the aid of these screw drivers, while observing it in the light microscope, relatively to the cylndric jacket 5l until it lies concentrically to the contact surface of the jacket. 64 and 65 denote springs allotted to the screw drivers for pressing the screw drivers in the Operating position against the respective set screws.

After the cathode point i has exactly been centered, the glass member 45 is firmly secured to the metallic sleeve with the aid of a suitable cement so that a shifting of the centered cathode point is no longer possible. The cathode insert may be inserted in the diaphragm cap 2 after centering the cathode point by securing the jacket together with the insert to the cathode shaft 'I in the manner shown in Fig. 2. Instead of a cathode insert having a metallic jacket as shown in Figs. 2 to 4, an insert may be employed which consists completely of a ceramic or similar material.

What is claimed is:

1. In an electron-emitting apparatus for highvoltage operation, a cathode structure, an anode structure having a centrally apertured anode diaphragm, and a. hollow insulator mechanically connecting said cathode structure with said anode structure, said cathode structure comprising a tubular casing arranged within said insulator, a diaphragm cap detachably mounted on the end of said casing facing said anode diaphragm, and a detachable insert provided with a cathode proper, said insert being arranged in fixed relation to said casing and secured thereto by said diaphragm cap.

2. In an electron-emitting apparatus for highvoltage operation, a cathode structure, an anode structure, and a hollow high-voltage insulator vacuum-tightly enclosing said. cathode structure and mechanically connecting it with said anode structure, said cathode structure comprising an elongated tubular casing, a detachable insulating insert secured to said casing at the end of said casing racing said anode structure, an incandescent cathode propermounted on said insert, a centrally apertured cap-shaped member detachably secured to said casing so asto cover said cathode proper, and conductors for supplying current to said cathode proper, said conductors being arranged in said tubular casing, and said casing having lateral openings connecting its interier space containing'said conductors with the surrounding vacuum space within said insulator.

3. In an electron-emitting apparatus for highvoltage operation, a cathode structure, an anode structure, and a hollow high-voltage insulator vacuum-tightly enclosing said cathode structure and mechanically connecting it with said anode structure, said cathode structure comprising an elongated tubular casing, an insulating insert detachably secured to said casing adjacent to said anode structure, a cathode proper mounted on said insert so as to face said anode structure, two plug-contact members mounted on the other side of said insert and electrically connected with said cathode proper, a centrally apertured and cap-shaped diaphragm detachably arranged on said casing so as to hold said detachable insert firmly in proper position relative to said casing,`

',. nently secured in said casing, an insert of insulating material detachably securedto said casing adjacent to said anode structure, means for holding said detachable insert in a given centered position relative to said casing, a hot cathode proper mounted on said insert facing said anode structure and laterally displaceable relative to said insert, and adjusting means dispesed on said insert and connecting said insert With said cathode proper for adjusting the position of said cathode proper in transversal directions relative to insert.

5. In an electron-emitting apparatus havinga cathode structure and an anode structure arranged in fixed relation and substantially coaxial to each other, said cathode structure comprising a tubular casing, current supply conductors disposed in said casing, an insert of insulating material detachably secured to said 'casing adjacent to said anode structure, means for holding said d-etahable insert in a given centered position relative to said casing, an insulating base member of inorganic refractory material slidably` mounted on said insert on the insert surface facing said anode structure, a hairpin-shaped cathode element mounted on said base member, means connecting said cathode element with said conductors, and adjusting screws arranged on said insert and bearing against said base member for shifting and adjusting said base member in transversal directions relative to said. insert.

6. In an electron-emitting apparatus having a cathode structure and an anode structure arrangecl in fixed relation and substantially coaxial to each other, said cathode structure comprising a tubular casing, current supply concluctors dissaidposed in said casing, an' insert of insulating material, said insert having a cylindrical metallic jacket, a cap-shaped member cletachably secured to said. casing and having an inner cylindric surface fitting said cylindric jacket to firmly secure said insert to said casing in a centered position, an insulating base member of inorganic refractory material slidably mounted on said insert acing said anode structure, a cathode proper mounted on said base member, contact means connecting said cathode proper with said conductors, and adjusting means Secured to said jacket and bearing against said base member at three points spaced angularly and evenly with respect to one another, said adjusting means comprising at least two set screws for shifting and adjusting said base member in any transversal direction relative to said insert. i i

7.`An auxiliary device for adjusting the detached insert of an electron-emitting apparatus as set forth in claim 6 with the aid of a lightoptical microscope comprising a holder having a cylindric bore coaXial to the optical axis of the light microscope and closely fitting the said bore to permit actuating said screws by said i screw drivers while observing the adjustment through said light microscope.

8. An auxiliary device for adjusting the detached insert of an electron-emitting apparatus as set forth in claim 6 with the aid of a lightoptical microscope, comprising a holder having a cylindric bore coaxial to the optical axis of the light microscope and closely fitting the aforesaid cylindric jacket of said insert, screw drivers arranged on said holder in permanent association therewith and spaced evenly from one another around said bore, and a spring disposed between said holder and each of said screw drivers for holding said drivers in engagemert with the aforesaid set screws of the insert inserted in said bore to permit actuatng said screws by saidscrew drivers while observng the adjustment through said light microscope.

9. The method of preparing exchangeable cathode inserts of electron-em'tting apparatus carrying a cathode proper adjustable transversally to the axis of the insert, which comprises heating said cathode proper outside of said apparatus in vacuum by a heating current of higher intensity than the normal Operating current, and adjusting the position of said cathode proper :relative to said insert so as to place said cathode ;proper in correctly centered position before inserting said insert into said apparatus.

10. In an electron-optical apparatus, an electron-emitting device for high Operating voltages comprising in combination an elongated cy1in`- drica'l cathode structure, an anode'istructure coaxial .with said cathode s'tructure,:a'n.d an iri-sulator having a cylindrical bore containing said cathode structure and being' mounted on said anode structure, said cathode structure and said nsulator having two coaxial and conical sur faces engaging each other to form a rigid and sealed joint, and said anode structure and said insulator having a second pair of coaxial. and conical surfaces also engaging each other to form a rigid and sealed joint, whereby said cathode structure, anode structure and insulator though detachable from one another are firmly secured together, with said cathode andanode structures substantially coaxial to the optical axis of the apparatus, said 'cathode structure including a detachable insulating insert a rranged adjacent to said anode structure and closely fitting the adjoining portion of said anode structure, and a cathode proper arranged'on said insert so as to be displaceable transversally to said optical axis, and adjusting means disposed on said insert and connecting said insert with said cathode proper for Varying and fixing the position of said cathode proper relative -to said insert to finely center said cathode proper.

11. With an electron-optical apparatus having a vacuum vessel, in combination, an electron source for high D. C. voltages comprising a highvoltage insulator having a cylindrical bore and two outer` conical sealing surfaces concentric to said bore at both ends respectively, an anode structure connected with the adjoining portion of 'said vacuum vessel of the apparatus and forming a base for supporting said insulator, said anode structure having a central anode diaphragm and an outer cylinder, said cylinder having an inner conical surface'engaging one of said conical surfaces of said insulator, and a cathode structure arranged substantially within said bore of said insulator and coaxially with said anode structure, said cathode structure comprising a tubular casing, a cap-shaped member firmly mounted on one end of said casing and having an inner conical surface engaging said other conical surface of said insulator, a cap-shaped and centrally perforated diaphragm detachably mounted` on the other end ofsaid tubular casing so as to lie adjacent to said anode diaphragm, and a removable cathode insert provided with the cathode proper and oovered by said diaphragm so as to be secured to said casing by said diaphragm, saidcap-shaped member and said cap-shaped diaphragm having cylindrical surfaces engaging said tubular member and concentric to said conical surfaces so that all of the aforesaid parts of saidcathode and anode structures are substantially in axial alignment with one another. 4

ERNST RUSKA. HANS SCHUCHMANN. EMIL HENTSCHEL; 

